When to Use a Rigid-Flex PCB vs. Multi-Board PCBs

 Printed circuit boards (PCBs) are at the heart of electronics devices, providing mechanical support and electrical connectivity between components. PCB design approaches balance tradeoffs across size, complexity, performance, and manufacturability. Engineers must choose between integrated rigid-flex PCBs or multi-board architectures based on product requirements. This article covers guidelines on selection criteria, capabilities, and implementation considerations when deciding between rigid-flex PCB and multi-board design.

Rigid-Flex PCB Overview

A rigid-flex PCB combines stiff “rigid” board sections interconnected by thin, bendable “flex” sections etched from flexible polymer materials. An example rigid-flex design is shown below:

Rigid sections provide mechanical structure while flex allows 3D configuration

Rigid-flex PCB technology integrates interconnection between areas needing connectors on separate PCBs into one monolithic structure. The technology provides:

• Flexible form factor – bend and fold to complex 3D geometric shapes
• Dynamic flexing – high fatigue life supports continuous motion
• Reliability – avoids connector failures prone with separate PCB stacking
• Miniaturization – fits complex electronics into small spaces

Multi-Board PCB Overview

Using individual PCBs for sub-systems interconnected by cables and connectors is a traditional “multi-board” design approach. For example:

Multi-board design with connectivity between PCBs via cables or connectors

This decoupled strategy allows independent development and layout of separate PCB functions but incurs penalties around:

• Size/weight – connectors, cages, cables, spacing add bulk
• Reliability – interconnects prone to vibration failure over time
• Performance – reflections degrade high speed signals

Understanding capabilities and tradeoffs guides appropriate application of each approach.

Rigid-Flex PCB Capabilities

Advances in rigid-flex materials, fabrication processes, and design tools over the past decade have greatly expanded the application space for rigid-flex PCBs across many industries.

Stacked Configurations

Rigid-flex allows stacking PCBs vertically by wrapping flex layers. Stacking reduces overall product footprint.

Rigid sections interconnected in vertical stack by folded flex

Dynamic Flexing

High flex life supporting millions of bend cycles enables continual motion. Useful in applications like printers.

Embedding Components

Components embed inside flex layers, saving space. Allows embedding transistors, capacitors, resistors.

High Speed

Matched impedance in flex enables multi-gigabit data rates. Useful for high speed compute and network fabrics.

Ruggedization

The flexible layers absorb shock/vibration instead of transferring energy into solder joints which reduces failures.

Miniaturization

Thin, tight, 3D packaging impossible with rigid-only PCBs. Enables product portability.

With these enhanced capabilities, rigid-flex PCBs now serve advanced applications in aerospace, defense, computing, autos, medical, robotics industries.

Multi-Board Systems Capabilities

While rigid-flex PCBs provide extremely dense integration, multi-board systems have complementary advantages:

Partitioning Complexity – Separating functions across boards splits layout complexity and allows independent test.

Routing Simplicity – Routing high pin count connections easier with multiple layers vs. single board.

Reuse – Reusable PCB modules lowers cost, risk.

Heat Dissipation – More surface area spreads heat vs. confined rigid-flex package.

Easier Field Servicing – Technicians can rapidly swap modular boards.

Technology Mixing – Different or incompatible component types partition onto separate boards.

Cost Savings – Small PCBs avoid expensive large board fabrication.

Determining the right balance of integration vs. modularity is key in selecting between approaches.

Rigid-Flex Implementation Considerations

While rigid-flex PCBs provide tremendous size and reliability benefits, engineers must weigh several considerations when applying the technology:

Design Overhead

Rigid-flex design complexity around Constraint Managers, layer transitions, and flex modeling represents initial overhead.

Fabrication Lead Time

Specialty materials and processes increase rigid-flex fabrication time from weeks to months depending upon complexity.

Cost

NRE cost for an equivalent rigid-flex PCB is higher than simpler rigid boards or multi-board designs. However, this offsets cable, connector, and manual assembly expenses later on.

DFx Challenges

Laminate transitions require specialist DfM review. Re-work and panelization restrictions also apply.

Qualification Strategy

The custom nature and new materials used in rigid-flex PCBs demand an extensive qualification test plan to productize reliably.

By planning ahead for these considerations, engineers can fully leverage rigid-flex benefits.

Decision Guidelines on PCB Approach

So when should engineers choose an advanced rigid-flex PCB architecture over traditional multi-board? While each product presents unique considerations, the following guidelines provide a starting point for technology selection:

<table> <tr> <td colspan="3"><b>Decision Guidelines on PCB Approach</b></td> </tr> <tr> <td><b>System Attribute</b></td> <td><b>Favors Rigid-Flex PCB</b></td> <td><b>Favors Multi-Board</b></td> </tr> <tr> <td>Size / Weight Budget</td> <td>Extreme space and weight constraints</td> <td>Standard form factors</td> </tr> <tr> <td>Reliability Requirements</td> <td>Very high MTBF essential</td> <td>End product MTBF less critical</td> </tr> <tr> <td>System Complexity </td> <td>Integration or partitioning flexibility needed</td> <td>System functionality well defined</td> </tr> <tr> <td>Schedule Timeline</td> <td>Time to market urgency</td> <td>Multi-phase project</td> </tr> <tr> <td>Production Volume</td> <td>High volume cost reduction essential </td> <td>Lower production quantity</td> </tr> <tr> <td>Serviceability </td> <td>Whole system disposable</td> <td>Field repair required </td> </tr> </table>

Product size, reliability, complexity, schedule, volume, and service parameters all factor into the decision between advanced PCB technologies.

Case Study Comparisons

Looking at examples of actual products highlights how the guidelines apply for different applications:

Wearable Health Monitor

This miniaturized design monitors vital signs:

<table> <tr> <td><b>Multi-Board Approach</b></td> <td><b>Rigid-Flex Approach</b></td> </tr> <tr> <td> <ul> <li> Separate PCBs for MCU, sensors, radio, power</li> <li>Interconnected by cabling and connectors </li> <li>Heavy and bulky relative to body mounting</li> </ul> </td> <td> <ul> <li>Integrates functionality into single thin PCB </li> <li>Lightweight form fits to arm </li> <li>Flex allows freedom of motion </li> </ul> </td> </tr> </table>

Wearability demands drove ultra-compact, reliable rigid-flex integration

Satellite Modem

The satellite link modem provides environmental resilience:

<table> <tr> <td><b>Multi-Board Approach </b></td> <td><b>Rigid-Flex Approach</b></td> </tr> <tr> <td> <ul> <li>Separated RF, DSP, and power boards</li> <li>Inter-